SECT, ii PHANEROGAMIA 551 



stigmatic surface (h) corresponding to two confluent stigmatic lobes. The third stig- 

 matic lobe is transformed into a structure termed the rostellum (I, k) and stands in 

 relation to the male organ. The single fertile anther consists of two thecae joined 

 together by the connective which appears as the end of the gynostemium. The 

 whole mass of pollen of each of the two pollen-sacs is joined together by an inter- 

 stitial substance which continues below to form a stalk ; the whole structure, which 

 has a waxy consistence, is called a pollinium, and the stalk goes by the name of 

 the caudicle. The caudicles terminate below in contact with the rostellum which 

 forms tough adhesive discs. This relation to the rostellum serves to keep the 

 pollinia, which lie free in the pollen-sacs, in position, and the adhesive discs attach 

 the pollinia to any body that comes in contact with them. If an insect alights on 

 the lower lip and attempts to reach the nectar secreted in the spur, its head or 

 tongue must touch the rostellum and the pollinia will become attached to it. As 

 the adhesive discs dry they cause the pollinia to bend forward, so that when the 

 insect visits a second flower they will be brought in contact with the stigmatic 

 surfaces. 



All Orchids are similarly adapted to insect visitors, though in many the con- 

 trivances are far more complicated ; pollination does not take place in the absence 

 of the insects. It should be mentioned that in some forms, e.g. Vanilla, the pollen 

 remains powdery. Many tropical Orchids are cultivated in greenhouses on account 

 of the beauty of their flowers. 



SUB-CLASS II 



Dicotylae ( 14 ) 



The Dicotyledons with few exceptions possess a pair of seed- 

 leaves ; these on germination either expand as green assimilating 

 leaves or remain within the seed-ctat and supply the seedling with 

 the reserve materials stored in their cells. The growing point of the 

 stem, lying between the cotyledons, grows into the shoot of the 

 seedling. The main root of the embryo has meanwhile penetrated 

 into the soil ; as a rule it persists as a tap-root and gives rise to a 

 regularly branched root-system. 



The stem has a circle of open vascular bundles, while the root on 

 transverse section shows a regularly alternating arrangement of the 

 xylem- and phloem-groups. The primary meristem situated in the 

 vascular bundles of the stem, or to the inner side of the phloem in 

 the root soon becomes completed across the medullary rays and forms 

 a complete, meristematic ring. By means of this cambium a regular 

 growth in thickness of the stem and root takes place (cf. Figs. 123- 

 124, pp. 112, 114; Fig. 128, p. 118; Fig. 138, p. 129; Figs. 146- 

 154, pp. 138-144). 



The typical form of leaf found among Dicotyledons is provided 

 with a longer or shorter petiole, and often has a pair of stipules 

 developed from the leaf-base ; a leaf-sheath is usually absent. The 

 lamina may be simple or compound ; the latter condition is always 



2N 1 



